This disclosure is directed to a centralizing apparatus adapted to be attached or affixed to a supported downhole tool. The precise nature and definition of the downhole tool is not particularly important, and may be by means of example and not limitation, an acoustic bond logging apparatus. Other types of downhole tools may be set forth, and they are generally described as including an elongate cylindrical housing which functions either as a central mandrel or which includes a central mandrel enabling it to be connected with a mandrel through the centralizer of this disclosure. They are connected serially to thereby define a mandrel supported centralizer biased by spring mechanisms to be described so that the bias force is approximately uniform over a range of deflection of the centralizer arms.
A centralizer is a device which is adapted to be connected serially with a downhole tool. Typically, a downhole tool is run in a well bore supported on a wireline. The centralizer is included to position the downhole tool at a central location in the axis of the well borehole. This is desirable ordinarily to prevent snagging the tool and to enable the procedure conducted by the downhole tool to be carried out in ordinary course. Moreover, such a centralizer tends to deflect, thereby repositioning itself and the attached tool so that they are near the centerline than would be the case without a centralizer. While in theory, the well borehole is round, in practice, there is a tendency for the downhole tool to decentralize, meaning that it is positioned against one side of the well borehole. This interfers with its operation, and creates problems in that the procedure conducted by the tool may be less than perfectly performed; even worse, decentralization is difficult to measure because of the randomness of such occurrences. This apparatus is a linear force centralizer which aids and assists in maintaining a supported tool in a more or less centralized position. In typical circumstances, duplicate sets of this equipment are installed with a downhole tool to aid and assist in positioning various portions of the tool body supported thereby central of the well borehole.
The linear force centralizer of this disclosure particularly features a linear resistent force acting on the extended arms. It includes preferably opposing pairs of arms. In fact, each arm is made up of first and second arm segments which are dissimilar in length. One is longer than the other and they join at a knuckle joint having a outwardly protruding roller to permit contact against the well borehole without frictional dragging wherein the roller protects the joined long and short arms. The arm segments are themselves connected to spaced crosshead assemblies. The crosshead assemblies support several such arms as for instance, four arm segments on each crosshead assembly to thereby define four pair of arms; opposite arms are symmetrical to one another while the remaining opposite pairs are also symmetrical; however, they are arranged to that their long arm segments are connected to the common crosshead with the short arm segments of the other pair. In other words, the four sets of arms that protrude radially outwardly are staggered with regard to their outwardly disposed contact means.
This staggered arrangement in conjunction with the equal force resilient spring means to be described below enables the system to form a restoring force. The equal force system refers to a set of springs which are arranged so that the arms are under approximate constant force with deflection. As they deflect through a specified angular range, the forces acting on the arms are approximately equal and hence balanced. This enables the arms to deflect through a relatively wide range without having a biased force. This assists in accommodating variations in borehole geometry. It also assists in providing a relatively constant centering force acting on the decentralized equipment when that occasion arises.
While the foregoing is generally descriptive of the background of the present disclosure and reveals few details of the present apparatus, the scope is set forth in greater detail hereinafter with the description of the preferred embodiment in conjunction with the below included drawings.